THE ROTIFERA. 167 



disk. The walls of this cavity are abundantly ciliated, and 

 at the bottom is a muscular pharynx, or mastax, provided 

 witli a peculiar armature. Sometimes, as in JStephanoceros, a 

 large crop-like cavity lies between the mouth and the mastax, 

 and the aperture of communication between this crop and 

 the mouth is guarded by a valve formed by two broad mem- 

 branous folds which project into the cavity of the crop. The 

 armature of the mastax generally consists of four pieces — two 

 lateral, the mallei, and two central, constituting the incus. 

 The contraction of the muscular masses, to which the mallei 

 are attached, causes the free ends of the latter to work back- 

 ward and forward upon the incus, and crush the prey which 

 is taken into the mouth. 1 



A short oesophagus, provided with cilia or vibratile mem- 

 branes, leads into a digestive cavity bounded by the enclo- 

 derm. The anterior or gastric part of this cavity is usually 

 dilated, and gives off a large caecum on each side. The pos- 

 terior, narrower, intestinal part usually opens externally by a 

 cloacal chamber ; but, in some Rotifers (e.g., Notommatd), 

 the alimentary cavity is a blind sac, devoid of intestine or 

 anus ; and in the males, so far as they are known, the whole 

 alimentary canal is aborted and represented by a solid cord. 



A spacious perivisceral cavity occupies the interval be- 

 tween the walls of the alimentary canal and the parietes of 

 the body. The latter contains circular and longitudinal mus- 

 cular fibres, which may be smooth or striated. 



Opening into the cloaca there is usually a large thin-walled 

 vesicle with rhythmically contractile walls ; and, in connection 

 with this, are two delicate water-vessels, which pass forward, 

 often giving off short lateral branches, and eventually break 

 up into numerous ramifications in the trochal disk. The 

 branches are open at the ends, w r hereby the cavities of the 

 water-vessels are in communication with the perivisceral cav- 

 ity on the one side, and with the surrounding water on the 

 other. Here and there, in the course of the main trunks and 

 at the ends of the branches, long cilia, which, by their con- 

 stant undulation, give rise to a nickering motion, are situated. 



The nervous system is represented by a relatively large 

 single ganglion placed on one side of the body, near the tro- 

 chal disk. One or more eye-spots are sometimes seated en 

 the ganglion, and there are other organs which appear to be 



1 See, for the various forms of this apparatus, Gosse, "On the Structure, 

 Functions, and Homologies of the Manducating Apparatus in the Rotifera." 

 (Philosophical Transactions, 1855.) 



